Heat the butter over medium heat just until it comes to a boil. Remove from the heat and immediately add the sugar, cocoa powder, and dark chocolate. Mix together until combined. Let cool for 5 minutes.
In a small bowl, whisk together the eggs, yolks, and vanilla. Slowly add the eggs to the butter/cocoa/chocolate mixture from step 1.
Add the flour and salt and mix just until combined. Finish by mixing in the chocolate chips.
Spread into a 13x9 inch that has been prepped with nonstick spray and a sheet of parchment paper on the bottom.
Bake at 350F (180C) for 25-30 minutes or until a toothpick inserted in the center comes out with streaks of thick batter (the toothpick should not come out clean nor should it come out entirely coated with shiny, wet batter).
